Field of the Invention
The current invention relates generally to the production of colour images for such applications as displays and anti-counterfeiting, and in particular to the photolithographic fabrication of metallic nanostructures that transmit or reflect colour by plasmonic resonance.
In today's colour printers, colours are rendered by colour dyes that are deposited onto different positions on a substrate such as paper. In such a system, a range of colours is achieved by combining two or more colour dyes in different proportions. Colour printers have mechanical and/or optical systems that can position dyes accurately to achieve colour pixel sizes as small as several microns. However, the need for more than one material to be deposited requires reservoirs of the various colour dyes, in the form of cartridges. Furthermore, the colours achieved by mixing dyes of fixed light-absorption wavelengths are not as spectrally pure as those achieved by tuning the absorption wavelengths of materials, which could produce colours more vibrant to the eye. Finally, the resolution of the images produced is limited to the smallest amount of dye that can be deposited onto the substrate, typically microns in size. Industrial techniques such as inkjet and laserjet methods print at sub-10,000 d.p.i. resolutions because of their micrometer-sized ink spots. Research-grade methods are capable of dispensing dyes at higher resolution but are serial in nature and, to date, only monochrome images have been demonstrated. Plasmon resonances in metal nanostructures have been used to create colours in stained glasses since the 4th century AD. Plasmon resonances in metal films have also been used in macroscopic colour holograms, full colour filters and polarizers. The colour filters in particular exhibit the phenomenon of extraordinary optical transmission (EOT) and effect of Fano resonance through periodic subwavelength holes in the film. The colours produced are set by the periodicity of the structures, which are typically in the range 100-1000 nm, so multiple repeat units are required, resulting in relatively large, micrometer-sized pixels. In an alternative arrangement, small (tens of nanometers) isolated metal nanoparticles can be used, which scatter colours depending on their shapes and sizes, but do not scatter strongly enough to be viewed plainly in a microscope, especially when deposited in direct contact with a substrate.
Colour micro-images based on plasmon resonances are described in the prior art. For example, an aluminium film patterned with arrays of nanoholes is viewed in transmission using grazing angle dark-field microscopy and depending on the geometry of each array, different colours are observed (D. Inoue et al, “Polarization independent visible color filter comprising an aluminum film with surface-plasmon enhanced transmission through a subwavelength array of holes”, Applied Physics Letters 98, 093113 (2011)).
A method has recently been demonstrated for printing full-colour images at the optical diffraction limit by encoding colour information into silver or gold nanodisks formed on the tops of nanopillars above a back-reflector (Kumar et al., “Printing colour at the optical diffraction limit”, Nature nanotechnology, Vol. 7, pp. 557-561 (2012)). Electron beam lithography is first employed to write an array of nanoposts into a layer of negative-tone hydrogen silsesquioxane resist on a silicon substrate. The electron beam is used to define the diameters and separations of the nanoposts across the array so that the desired variation of colour is produced in the image. A 1 nm-thick chromium adhesion layer, a 15 nm-thick layer of silver or gold and finally a 5 nm-thick capping layer of gold are successively deposited onto the tops of the posts and onto the surface of the substrate. The coating on the latter acts as a back-reflector, which improves the efficiency of colour generation.
The different sizes and separations of the silver or gold nanodisks thus formed determine the interplay of plasmon and Fano resonances, and consequently the resulting colour. The images can be viewed in reflection under a bright-field optical microscope. It is additionally disclosed that the colours are preserved when only four nanodisks are present in each 250×250 nm pixel, thus enabling colour printing at a resolution of 100,000 dots per inch (DPI).
In another fabrication procedure a master mold comprising an array of nanoholes in silicon is first formed using electron beam lithography (Clausen et al., “Plasmonic metasurfaces for coloration of plastic consumer products”, Nanoletters, Vol. 14, pp. 4499-4504 (2014)). The sizes and distribution of the holes in the array are defined using the electron beam so that they correspond to the arrangement of colours in the desired image. The mold is then employed in a hot-embossing or an injection molding process to produce an array of nanopillars in a polymer material. An aluminium layer is subsequently deposited on the structure to form nanodisks on the tops of the pillars and a reflective layer around the pillars, and then a transparent protective layer is coated over the complete structure. It is described that the complete spectrum of colours may be produced by varying just the diameter of the nanodisks, without changing the array period. Using this procedure many replicas of the nanopillar structure may be formed in the polymer material from the master mold, and then aluminium deposited on the replicas simultaneously, thereby enabling a relatively low cost process for mass producing a particular image composed of array of metallic nanostructures.
The above prior art describes methods for fabricating metallic nanostructures that generate colour images by the mechanism of plasmonic resonance. The encoding of the different colours in the sizes and separations of the metallic nanodisks, however, rely on the expensive technique of electron-beam lithography. Whereas this is not necessarily a problem for mass producing a particular colour image that is not too large, when an embossing or injection molding process may be used to reproduce a nanopillar array, it does, however, represent a severe problem for producing different colour images on a commercial basis if only one or a small number of each are required. This would generally be the case for, for example, security applications. Also, if the required image size is large, such as 10 cm×10 cm or 50 cm×50 cm, then the cost for producing a master mask using electron-beam lithography can be prohibitively large.
Plasmonic resonance is also being investigated for other fields, for example, colour filters for OLED displays and CMOS image sensors. For these applications large arrays of metallic nanostructures are similarly needed that have to be formed very uniformly over the filter area. To achieve this in a cost-effective way, holographic interference lithography has been employed to form firstly uniform arrays of nanoholes in photoresist layers on aluminium-coated substrates (J-H. Seo et al., “Nanopatterning by laser interference lithography: applications to optical devices”, J. Nanosci. Nanotechnol., Vol. 14(2), pp. 1521-32 (2014)). In this procedure, a two-beam interference system exposes the photoresist to a 1-D line-space pattern in a first exposure, the substrate is then rotated by 90°, and the 1D pattern exposed again on top of and orthogonal to the first exposure. The superposition of the two exposures produces the desired two-dimensional array of holes in the developed photoresist. The structure is then etched to transfer the pattern into the underlying aluminium. The colour of the resulting filter is determined by the diameter of the holes, which may be enlarged or reduced by adjusting the dose of the combined exposure. Whereas this exposure technique is suitable for fabricating colour filters, it is totally unsuitable for producing colour images because of the lack of control available for adjusting the relative sizes or separations of the nanostructures in different pixels for obtaining the required colour variations across the image.